Every Android developer has run into this issue on more than one occasion: One minute everything is working fine,
the next minute Eclipse is lit up like a Christmas tree. Nearly every module in your project
fails to compile, and you’re seeing red. Before you even look at the problem pane, you probably know that you are going to see “R cannot be resolved.”

This is one of the most frustrating and common issues with the Android
development platform. If you
don’t believe me, google the subject — you’ll find pages of developers asking for help and others offering to help solve this infuriating problem.

If there is already an abundance of suggestions
and tips for resolving this issue, you might be wondering why I felt the need to write this article. Well, in my experience, there
are numerous causes for this error, and sometimes it takes a minute to correct the error, and other times it takes
hours to resolve it. Every time I comb the Internet posts, I read the same posts over and over and pick out one suggestion from myriad threads and sites. When everything is finally up and
running again, I wonder why someone hasn’t written a consolidated
troubleshooting guide for this error. I decided to do it.

So without further ado, here is an exhaustive guide to
getting your project back up and running the next time Eclipse bombards you
with “R cannot be resolved” (Figure A). If
you notice a scenario I missed, please let me
know in the discussion thread. I
will see that it is added to the troubleshooting guide, and that you get attribution for the tip.

Figure A (See an enlarged view of this image.)

1:  Clean, clean, and clean

I know this sounds obvious, but you’d be
surprised at how often this is the cure. In particular, if you recently refactored something like a package
name, many times just a clean will get you back up and running. The suggestion that seems to work best
in most cases is to turn off auto-build in Eclipse, do a clean of the entire
project, and then re-enable auto build.

2: Disable Eclipse “auto imports” and “auto
organize inputs on save”

Whenever your project has failed to generate the R file, turning Eclipse “auto imports” and “auto organize inputs on save” options off is a must (don’t worry — you don’t have to leave these options permanently disabled). The reason is that once Eclipse can’t resolve R it will sometimes add an “Import Android.R” to the top of your files that use resources. This is not what you want and, once it
happens, you’ll have to manually go through your source files, check for the bad
import, and remove it by hand.

3: Check the /res directory for malformed XML

Eclipse is a big bulky environment, and
sometimes it’s slow to catch an error. There are many times I’ve created an XML resource, saved the file, and closed it without noticing any complaints from Eclipse.  However, when the ADT finally does validate the XML, the generation of the resource file stops, and none of
the source modules can locate the R file. As a matter of protocol, whenever I encounter this error, I open my entire
resource tree in Eclipse and re-examine the folders / files for the red “X” indicating an error slipped by me.

4: Comply with Android file naming conventions

Android does not allow certain characters
in file names that other development platforms do. Two that have stumped me on more than one occasion are the “dash” character and, wait for it, any uppercase character. File names for resources in an Android
project may consist of lowercase letters, a period, and the underscore only. Any attempt otherwise will result in
the platform failing to generate your resource package.

5: Double check your resource package name in the
manifest

If you’ve gotten this far and still
have not resolved the error, open your AndroidManifest.xml file and verify that you didn’t inadvertently change your project’s package
name: <manifest
xmlns:android=”http://schemas.android.com/apk/res/android”
package=”your.correct.package.name” … .

6: Remove the “app_name” from the
strings.xml file

While having an “app_name” in the strings
file is not mandatory, the default manifest Android’s project wizard creates a reference
to it. My suggestion is do not remove the entry, but if you do, make sure to update the manifest to match.

7: Make sure you know your min and target SDK

Android is an evolving platform, and over
time commands and keywords have been added. If you recently changed your target build, min SDK, or
updated the tools, try rolling back to the previous
SDK you were using by bringing up your project properties and selecting
the previous SDK.

8: Are you using a version of the ADT greater than
or equal to 22?

This only became an issue
when Google rolled out its new Android Development Studio. Beginning with ADT 22, there is something called Android SDK Tools, and this package doesn’t always
seem to get installed as part of the auto update process.

To find out if you may be a victim of
the persnickety installer, use the wizard in Eclipse to create a brand new “Hello World” application. If this
project cannot generate R, there is definitely something askew with your
environment. Open the SDK manager,
find the tools folder, and make sure SDK Tools, SDK Platform Tools, and SDK
Build Tools are all installed. If
not, you can check the boxes, apply the install, and then restart Eclipse.

9: Create a new workspace

It may become easier to create
a new workspace for your project than to continue to troubleshoot the existing one. I suggest trying to
import your project; if that doesn’t work, try creating a new project from the existing source. As a last resort, create an empty project and bring your application in piecemeal. This is time-consuming, but it should help
to point you to the file or files causing the error.